Fig. 1: The optical response of many semiconducting materials is described by the excitation of excitons.
From: Disentangling the multiorbital contributions of excitons by photoemission exciton tomography
a In the exciton picture, each excited state is described by an exciton energy Ωm, which (for bright excitons) can be measured by optical spectroscopy. b At the orbital level, however, each exciton is built up by an entangled sum of electron-hole pairs ϕvχc (see Eq. (1)). In this description, the sum of orbital contributions provides complete access to the spatial properties of the exciton. c In photoemission exciton tomography, a high-energy photon photoemits the electron and thereby breaks up the exciton. The single-particle electron orbitals (here LUMO (L) and LUMO+1 (L+1)) contributing to the exciton are imprinted on the photoelectron momentum distribution, while the kinetic energy distribution probes the contributing hole orbitals by measuring the hole binding energy EH (see Eq. (2)). A full momentum- and energy- resolved measurement of the photoelectron spectrum therefore provides an ideal starting point for a comparison to ab-initio calculations of the excitonic wavefunction, and thereby provides access to the spatial properties of the exciton.
